In a wire electric discharge machining, if the machining is completed on all the commanded machining path, a male form drops to possibly collide with a lower wire guide and damage it. Especially in machining a plurality of forms from a single workpiece, when the machining of one form is completed, it is necessary to cut a wire electrode and move the workpiece to be positioned at a machining start hole for the next machining form, and to automatically connect the wire electrode through the next machining start hole. The dropped male form might slide on the lower wire guide in moving the workpiece to cause damage to the lower wire guide. Therefore, a first machining is performed on a great part of the machining form and is stopped immediately before the male form drops, to proceed to the next machining form with the male form remaining on it. Then, the electric discharge machining is performed on the next machining path to a position immediately before the male from drops. In a second machining, each male form is cut off the workpiece in the presence of an operator.
The following two methods are known for performing the second machining.
(1) The wire electrode is inserted through the machining start hole and is moved along the already machined groove with the electric discharge off (or with the electric discharge on), thereby cutting off the reserved section by the electric discharge machining. This method is adopted for a male form of relatively light weight. (2) After clamping the male and female forms so as to be bridged therebetween on the first machining path to hold the male form, the wire electrode is inserted through the machining start hole and then the workpiece is moved reversely in the second machining path to perform the electric discharge machining, thereby cutting off the reserved section.
In the method (1) of the above two methods, when the wire electrode is moved along the machined groove with the electric discharge off, the wire electrode might be stuck as it is caught by the workpiece. When the wire electrode is moved along the machined groove with the electric discharge on, the male form might be deformed as the electric discharge machining is performed twice. In the method (2), it is necessary to create a machining program to move the workpiece reversely. In either of the two methods (1) and (2), it is required to prepare two machining programs for the first machining and the second cutting-off machining.
Thus, in the foregoing conventional wire electric discharge machining methods, two programs are necessary for the first machining to leave a reserve section and the second machining to cut off a male form by machining the reserve section.